Linear, voltage variable, temperature stable gain control

ABSTRACT

An electronic gain control circuit having first and second differential amplifiers and an operational amplifier wherein the first differential amplifier is coupled to the input and provides the controlled output, the second differential amplifier is coupled to the first differential amplifier and controls the gain of the circuit, and the operational amplifier is coupled to the second differential amplifier for temperature stability and linearity.

United States Patent [191 Hughes 1541 LINEAR, VOLTAGE VARIABLE,

TEMPERATURE STABLE GAIN CONTROL [75] Inventor: Richard Smith Hughes,Ridgecrest,

Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy [22] Filed: Dec. 20, 1971 21 Appl. No.: 209,843

52 us. Cl. ..330/29, 330/30 D 51 Int. Cl. .1103; 3/30 58 Field of Search..330/23, 29, 309,

[56] References Cited UNITED STATES PATENTS 3,533,005 lO/l970 Aim..330/29 51 Apr. 10, 1973 Addis ..330/29 X 7/1964 Gruetman ..330/29Primary Examiner-Roy Lake Assistant Examiner-James B. Mullins Attomey-R.S. Sciascia et al.

[57] ABSTRACT An electronic gain control circuit having first and seconddifferential amplifiers and an operational amplifier wherein the firstdifferential amplifier is coupled to the input and provides thecontrolled output, the second difierential amplifier is coupled to thefirst differential amplifier and controls the gain of the circuit, andthe operational amplifier is coupled to the second differentialamplifier for temperature stability and linearity. I

3 Claim, 3 Drawing Figures PATENTED APR 1 01973 Vcc 'VEE Av=v -v FIG. 2.

PRIOR ART FIG. 'I.

PRIOR ART BACKGROUND OF THE INVENTION The present invention relates tothe field of gain control circuits. Amplifiers which electronicallycontrol their own gain are essential in almost all communicationreceivers, such as radar receivers. Although there are many priordevices which electronically control the gain of an amplifier, such asforward resistance, saturation resistance, channel resistance, photocellresistance, and transconductance devices, all except one suffer fromnonlinear gain change, temperature drift, and low range signal handlingcapabilities. The exception is the photocell resistance device which hasdisadvantages of its own, e.g., it requires relatively high power todrive the lamp and the cell must be shielded from ambient light.

The present invention overcomes the disadvantages of the prior devicesby utilizing the unique current-division property of the basicdifferential amplifier to provide a linear, temperature-stable,variable-gain amplifier which has a good control range, control/outputisolation, and large signal handling capabilities.

SUMMARY OF THE INVENTION The present invention is an automatic,electronic gain control having first and second differential amplifiersand an operational amplifier wherein, over a large dynamic range, thegain is not dependent on the signal amplitude. The first differentialamplifier is coupled to the input and provides a controlled output,'thesecond differential amplifier is coupled to the first differentialamplifier and controls the gain of the circuit, and the operationalamplifier is coupled to the second differential amplifier fortemperature stability and linearity. The invention utilizes the currentdivision characteristics of the differential amplifier to accomplishgain control by coupling the base of section B of the first differentialamplifier directly to the base of section A of the second differentialamplifier, a control voltage and the collector voltage of section B ofthe second differential amplifier to the inputs of the operationalamplifier, and the output of the operational amplifier to the commonconnected bases of section B of the first differential amplifier andsection A of the second differential amplifier. Thereby, the currentdivision ratio is dependent on the control voltage; and the gains of thedifferential amplifiers are mutually dependent. As a result, thecontrolled gain of the invention is linear and temperature stable.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of thebasic differential amplifier;

FIG. 2 is a schematic diagram of the basic, variable gain, differentialamplifier; and

FIG. 3 is a schematic diagram of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the basicdifferential amplifier which is well known and used extensively in thefield of electronics. The equations for the collector currents of thebasic differential amplifier, assuming where AV (V, V,), m q/KT, K isBoltzmanns Constant, q is the electron charge, and T is the temperaturein degrees Kelvin. Since I, is a constant, and

any decrease in 1,, corresponds to a like increase in I Dividing I by1,, the Current Division Ratio (CDR) is obtained:

The current source may be obtained with a common emitter transistor asshown in FIG. 2. If the input signal drives the common transistor Q thent tnl ei (3) where I, is the current through Q due to the input signal.The current I, will divide between 0 and Q The circuit gain for FIG. 2(assuming oz a, a, 1 may be expressed as 0! cr n- From Equation 4 andthe relationship ln e,

The gain, A is given by n c1/ e)- 02) Using the same procedure, the gainfor E may be found as Thus the gain for the amplifier of FIG. 2 isdependent on R R CDR and not upon the input signal being amplified.

A linear gain change characteristic can be obtained by making 1 or I alinear function of AV. This, and inherent temperature stability, can beobtained with the present invention, shown in FIG. 3. Transistors Q 0,,Q, and 0 may be matched integrated differential Thus I, V mm, AVG/R7,

and the collector current is a linear function of the control voltage.Since the current division ratio is dependent only on AV (Equation 4),

CDR 1 /1 15 Now substituting 1,, E V /R and Equation 14 into Equation15, we get If we let R equal R and add Equation 17 and Equation 18, wefind that v1 V2 a Equation 19 indicates that, as A increases, Adecreases by the same amount. Excellent temperature stability isobtained since the operational amplifierwill make sure that thecollector voltage of Q equals the control voltage. a

The gain-controlling amplifier section (Q Q and the operation amplifier)is ac-isolated from the gaincontrolled amplifier section (Q, and Q2).Thus the CDR (and the gain) is not affected by the input signal, whichcan be dc.

The dc collector voltages are dependent on the Current Division Ratio.If the control voltage is modulated, this modulation also appears at theac-isolated from the gain-controlled amplifier, V V If the outputs of Qand Q are used to drive a differential amplifier with gain A thecommon-mode dc voltage may be Y subtracted out.

when the collector voltage of transistor Q equals the control voltage Vthe output of the operational amplifier is discontinued and the bases oftransistors Q and Q, are no longer driven. As a result, current I, isgiven by Equation 14 and controlled by control voltage V the onlyvariable in the equation.

Therefore, since differential amplifiers l2 and 14 have inherent andequal current division ratios and are coupled together, and one of thecurrents (I is a linear function of the control voltage V the currentdivision, and thereby the gain, of the gain stage (differentialamplifier 12) is a linear function of the control voltage V The designof a complete amplifier becomes a relatively straightforward matter.Only two steps are required: first, determine the maximum gain (AV 0)and then detennine the gain slope (dAy/dAV Both can be determinedtheoretically from'the gain equation (Equation 1 8).

M J S (20 dA /dAV =R R5 m/ 7 EE) If a common-mode rejection amplifier isused on the output, its gain A multiplies Equation 20 and Equation 21.

What is claimed is:

1. An electronic device .for automatically controlling gain, comprising:

V an input;

a first differential amplifier coupled to said input for providing thegain controlled output of said device; and 1 gain controlling meanscoupled to said first differential amplifierfor controlling the gain ofsaid device and causing the output of said device to be linear andtemperature stable, including an operational amplifier providing anoutput descriptive of the difference between the instantaneous gain andthe preselected gain of the device,

a control voltage coupled to one input of said operational amplifier,

a second differential amplifier having at least one transistor, and

means for coupling the voltage on the collector of one of saidtransistors to another'input of said operational amplifier.

2. The device of claim 1 wherein: u said first and second differentialamplifiers are transistor circuits; and I the base of the secondtransistorof said first differential amplifier is direct coupled to thebase of the first transistor of said second differential amplifier. 3.The device of claim ZWhereinthe collector of the second transistor ofsaid second differential amplifier is coupled to another input of saidoperational amplifier and the output of said operational amplifier iscoupled to said direct coupled bases, such that the operationalamplifier provides a signal to the direct coupled bases in response tothe difference between the control voltage and the collector voltage ofthe second transistor of i i i i

1. An electronic device for automatically controlling gain, comprising:an input; a first differential amplifier coupled to said input forproviding the gain controlled output of said device; and gaincontrolling means coupled to said first differential amplifier forcontrolling the gain of said device and causing the output of saiddevice to be linear and temperature stable, including an operationalamplifier providing an output descriptive of the difference between theinstantaneous gain and the preselected gain of the device, a controlvoltage coupled to one input of said operational amplifier, a seconddifferential amplifier having at least one tranSistor, and means forcoupling the voltage on the collector of one of said transistors toanother input of said operational amplifier.
 2. The device of claim 1wherein: said first and second differential amplifiers are transistorcircuits; and the base of the second transistor of said firstdifferential amplifier is direct coupled to the base of the firsttransistor of said second differential amplifier.
 3. The device of claim2 wherein the collector of the second transistor of said seconddifferential amplifier is coupled to another input of said operationalamplifier and the output of said operational amplifier is coupled tosaid direct coupled bases, such that the operational amplifier providesa signal to the direct coupled bases in response to the differencebetween the control voltage and the collector voltage of the secondtransistor of the second differential amplifier.